Producing components from a hardenable steel, in particular hardened components, is known. Hereinafter, hardenable steel is to be understood to be steel in which a phase transition of the basic material occurs during heating, and in which a material, which is significantly harder or has higher tensile strengths than the starting material, results in a subsequent cooling, the so-called quench hardening, from the previous structural transformation and, optionally, further structural transformations during quench hardening.
For example, the method of the so-called press hardening is known from DE 24 52 486 C2, in which a plate of a hardenable steel material is heated to above the so-called austenitizing temperature and, in the heated state, is inserted into a forming tool and formed and simultaneously cooled in this forming tool, which on the one hand results in the final geometry of the desired component, and, on the other hand, in the desired hardness or strength. This method is widely used.
A method in which a hardened component is produced from hardenable steel sheet with a cathodic corrosion protection, in which the component is cold formed already in a metal-coated state so that it is 0.5% to 2% smaller than the nominal final dimension of the finished hardened component, is known from EP 1 651 789 A1. The component is then heated and inserted into a tool which corresponds exactly to the final dimensions of the desired component. The coated component has expanded to exactly this final dimension by thermal expansion, and is held on all sides and cooled in the so-called forming tool, which causes hardening to occur.
Moreover, a method is known from EP-A 0 971 044 in which a metal sheet from a hardenable steel and with a metallic coating is heated to a temperature above the austenitizing temperature and is then transferred into a hot-forming tool, where the heated metal sheet is formed and simultaneously cooled and hardened by the cooling process.
It is a drawback of the aforementioned methods for hot forming that—independent from whether or not there is a metallic coating on the steel substrate—micro-cracks occur in the steel substrate, in particular during hot-forming, but also in cold-preformed components, in which the forming process has not been completed.
These micro-cracks occur, in particular, in areas that are being formed, and in particular in areas with high degrees of forming. These micro-cracks are located on the surface and/or in the metallic coating and may partially extend relatively far into the basic material. In this case, it is disadvantageous that such cracks continue to grow if the component is subjected to stress, and that they constitute damage to the component that can lead to failure in the case of stress.
Metallic coatings on steel have long been known in the form of aluminum, aluminum alloy coatings, in particular aluminum-zinc alloy coatings, zinc coatings and zinc alloy coatings.
Such coatings have the purpose of protecting the steel material against corrosion. In the case of aluminum coatings, this is effected by means of a so-called barrier protection, in which the aluminum creates a barrier against the admission of corrosive media.
In the case of zinc coatings, protection is effected by means of the so-called cathodic effect of the zinc.
So far, such coatings have been used in particular in the case of normal-strength steel alloys, in particular for motor vehicle construction, building industry, but also in the household appliance industry.
They can be applied onto the steel material by hot-dip coating, PCD or CVD methods or by electrodeposition.
By using higher-strength steel qualities, an attempt was also made to coat the latter with such hot-dip coats.
From DE 10 2004 059 566 B3, for example, a method for hot-dip coating a strip of higher-strength steel is known in which the strip is first heated to a temperature of approximately 650° C. in a continuous furnace in a reducing atmosphere. At this temperature, the alloy constituents of the higher-strength steel are supposed to diffuse to the surface of the strip in only small quantities. The surface, which in this case consists primarily of pure iron, is converted into an iron oxide layer by a very short heat treatment at a higher temperature of up to 750° C. in a reduction chamber integrated into the continuous furnace. This iron oxide layer is supposed to prevent the diffusion of the alloy constituents to the surface of the strip in a subsequent annealing process at a higher temperature in a reducing atmosphere. In the reducing atmosphere, the iron oxide layer is converted into a purer iron layer onto which zinc and/or aluminum is applied in the hot-dip bath so as to adhere optimally. The oxide layer applied by means of this method is supposed to have a thickness of maximally 300 nm. In practice, the layer thickness is mostly set to approximately 150 nm.
It is the object of the invention to provide a method for producing hardened components from hardenable steel with which the forming behavior, in particular also the hot-forming behavior, is improved.
It is a further object to provide a steel strip which has an improved formability, in particular hot-formability.